1. Field of the disclosure
The present disclosure relates to a capacitive pressure sensor including a sensor chip having a diaphragm structure that detects a capacitance corresponding to pressure of a medium to be measured.
2. Description of the Related Art
In pressure sensors, including vacuum gauges which are used for example in semiconductor manufacturing facilities, a sensor element including a small diaphragm has been often adopted using a so-called micro-electromechanical systems (MEMS) technique. A main detection principle of this sensor element is that the pressure of a pressure medium is received by the diaphragm and the resulting displacement of the diaphragm is converted into some type of signal.
For example, as a pressure sensor that uses a sensor element of this type, a capacitive pressure sensor is widely known. The capacitive pressure sensor is configured to detect, as a change in capacitance, the displacement of a diaphragm that flexes in response to pressure of a medium to be measured (hereinafter referred to as “measured medium”). The capacitive pressure sensor is less dependent on the type of gas, and thus is often used in semiconductor facilities and various industrial applications. For example, this capacitive pressure sensor is used to measure a vacuum in a manufacturing process in a semiconductor manufacturing apparatus. The capacitive pressure sensor for measuring a vacuum is called a diaphragm gauge. The diaphragm that flexes in response to pressure of the measured medium is called a pressure-sensitive diaphragm or sensor diaphragm.
The diaphragm gauge is required to be resistant not only to corrosion by material gases and cleaning gases, but also to accumulation of byproducts produced during the process (hereinafter, these substances are referred to as “contaminants”). During normal operation in facilities, the accumulation occurs not only inside the process chambers, but also inside the pipes, pumps, and diaphragm gauge, and causes errors in vacuum measurement. To reduce unexpected accumulation of contaminants, vacuum components such as chambers, including the diaphragm gauge, are normally self-heated at a temperature of up to about 200° C. This means that the diaphragm gauge is required to be resistant to this self-heating temperature, as well as to the corrosion described above.
During the process or maintenance of the semiconductor manufacturing apparatus, a measured medium (e.g., gas) having a pressure within, or even beyond, the measurement range is repeatedly applied to the sensor element inside the diaphragm gauge in a self-heating state. Therefore, if there is residual stress produced during sensor manufacture, an error, such as hysteresis, appears in the output signal and this affects sensor accuracy.
As described above, it is essential for the diaphragm gauge to apply pressure to the diaphragm for the purpose of measurement. Pressure as high as atmospheric pressure, or the full-scale pressure of the sensor, is expected to be repeatedly applied to the diaphragm. Then, the sensor is normally evacuated again and returns to the zero point.
Hysteresis associated with a significant increase or decrease in applied pressure (i.e., hysteresis originating from pressure caused by changes in the state of mechanical stress) has been thought to occur at a joint portion of a sensor member. When pressure is applied, a metal housing of a package or the sensor member deforms, and its impact is transmitted to a diaphragm. This changes the shape of the diaphragm, and may cause hysteresis or shifts.
In many cases, this problem is created by a contact portion, such as a welded portion, which is susceptible to state changes resulting from deformation caused by pressure. Also, a joint portion where different types of materials are joined together may be irreversibly changed by a small mechanical impact, due to significant strain at the interface of the joint portion. An electrode material forming a capacitance may deform in response to pressure applied thereto. Preventing the occurrence of shifts or hysteresis in the region of pressure which can be applied to the sensor has been a great challenge in designing pressure sensors. Various measures have been taken to solve this challenge (see, e.g., Japanese Unexamined Patent Application Publication No. 9-61270).
In the diaphragm gauge, however, heat transfer from the measured medium to the diaphragm upon receiving pressure is essentially inevitable in measurement principle, and this poses challenges other than that described above. In particular, when there are local temperature changes in part of the sensor element (especially in a diaphragm region), such temperature changes directly lead to hysteresis errors in sensor output. To maintain the accuracy of the sensor product, it is important to minimize the impact of such temperature changes.
However, even when an attempt is made to control the self-heating temperature to achieve uniform temperature, such an attempt has no significant effect on the local temperature distribution inside the sensor in a vacuum state. This is probably due to the significant impact of heat transfer by gas molecules. As a measurement pressure range decreases, the level of hysteresis appearing in the output becomes more severe as a result of an increasing impact of errors on the full-scale pressure. Since this greatly affects the measurement accuracy of the sensor, an improvement needs to be made.
As described above, most measures that have been taken so far are related to dealing with residual stress produced during manufacture of the sensor structure. Currently, no sufficient measures are taken to deal with the factor (heat transfer from the measured medium to the diaphragm) which becomes apparent in a micropressure range and is inevitable in measurement principle.
This challenge will be specifically described with reference to FIG. 9. FIG. 9 illustrates a configuration of a main part of a diaphragm gauge according to the related art. A diaphragm gauge 100 includes a diaphragm unit 103 including a diaphragm (sensor diaphragm) 101 displaced in response to pressure of a measured medium and a diaphragm support portion 102 configured to support the periphery of the sensor diaphragm 101, a sensor base 105 joined to one side of the diaphragm support portion 102 and configured to define a reference vacuum chamber 104 together with the sensor diaphragm 101, and a base plate 107 joined to the other side of the diaphragm support portion 102 opposite the sensor base 105 and configured to define a pressure introducing chamber 106 together with the sensor diaphragm 101.
In the diaphragm gauge 100, a fixed electrode 108 is formed on a surface of the sensor base 105 adjacent to the reference vacuum chamber 104, and a movable electrode 109 is formed on a surface of the sensor diaphragm 101 adjacent to the reference vacuum chamber 104 in such a manner as to face the fixed electrode 108. The base plate 107 has a pressure introducing hole 107a in the center thereof (corresponding to the center of the sensor diaphragm 101). In the diaphragm gauge 100, the measured medium is introduced through the pressure introducing hole 107a into the pressure introducing chamber 106 and causes the sensor diaphragm 101 to flex.
When the diaphragm gauge 100 is used to measure a vacuum in a semiconductor manufacturing process, normally, the chamber is evacuated and the diaphragm gauge 100 maintains the zero point, except during processes under a predetermined set pressure or during maintenance which involves exposure to atmosphere. FIG. 10 schematically illustrates how the diaphragm gauge 100 is installed in a semiconductor manufacturing process. In FIG. 10, reference numeral 111 denotes a main chamber and reference numeral 112 denotes a pipe. At the beginning of measurement, a residual gas of the measured medium, produced during the previous measurement, is in the main chamber 111 and the pipe 112. Even in an evacuated state after application of full-scale pressure, the residual gas of the measured medium flows from the main chamber 111 through the pipe 112 while repeatedly colliding and exchanging thermal energy with the wall of the pipe 112, and eventually reaches the sensor diaphragm 101 in the diaphragm gauge 100.
At this point, if, before application of pressure, there is a difference between the temperature in the main chamber 111, the pipe 112, and the inner surface of the package of the diaphragm gauge 100 and the temperature in the vicinity of the sensor diaphragm 101, the temperature at the location where the gas initially reaches the sensor diaphragm 101 locally rises and falls by exchange of thermal energy. If the pressure range of an object to be measured is high, heat diffuses because the sensor diaphragm 101 is relatively thick, and local expansion does not occur or has a very limited impact.
However, in a micropressure range, the sensor diaphragm 101 is made thin to achieve higher sensitivity to pressure. Thus, heat does not diffuse, and a phenomenon of local expansion or contraction occurs. That is, as illustrated in FIG. 11, heat collects in the center of the sensor diaphragm 101, a microscale temperature distribution is produced, and a phenomenon of local expansion or contraction occurs.
If such local expansion or contraction occurs, the shape or the original and initial flexure of the sensor diaphragm 101, or the method of securing the sensor diaphragm 101, probably causes flexure which is independent of pressure, and generates shifts in sensor output. When a gas with a temperature different from that of the sensor diaphragm 101 is continuously supplied from the pipe 112 and the degree of vacuum in the background is, for example, about 10−4 Pa (0.001% FS of a 13-Pa range sensor), it takes a long time to uniformly distribute heat over the pipe 112 and the inner wall of the sensor through exchange of thermal heat by residual gas molecules. This means that a long and gradual shift is observed until the original state is restored. If the original state is not completely restored, the shift appears as an offset in the output.
The present inventors applied a constant heat flux to an area with a diameter of 2 mm in the center of a diaphragm having an initial flexure of 0.1 μm and a diameter of 7.5 mm, and calculated the level of temperature rise and flexure. When the thickness of the diaphragm was doubled (e.g., from 25 μm to 50 μm), the temperature rise was substantially halved and the flexure caused by the application of heat flux was reduced to a little more than one-eighth of the original. The present inventors thus found out that the phenomenon described above was less likely to occur in a thicker diaphragm.